The problem will firstly be explained with reference to self-energizing electromechanical brakes but relates equally to conventional electric brakes in which there is the risk that they will no longer release. This may be the case where the brake locks on account of a fault, for example a spindle breakage. Furthermore, there are electric brakes which can engage of their own accord—based upon energy stored in the vehicle or in the brake.
Self-energizing electromechanical brakes normally comprise an electrically actuated actuator, usually an electric motor, which presses a friction member with a friction lining against, and releases it from, an element, such as e.g. a brake disc, which is to be braked. When the brake is actuated, the friction member is carried along by the kinetic energy of the element to be braked and reinforces the braking effect automatically (self-energization).
Self-energizing electromechanical brakes are typically implemented as wedge brakes. Such a wedge brake is described e.g. in DE 198 19 564 C2 and is adequately known from many other printed publications. The wedge element is generally dimensioned such that the tangent of the wedge angle α corresponds approximately to an expected coefficient of friction μ between friction member and element to be braked. In this ideal case, the wedge element is neither pushed into nor pulled out of the gap so that the force to be applied by the actuator is equal to zero. Where coefficients of friction between friction member and element to be braked are unfavorable, i.e. high, on the other hand, relatively high tractional forces have to be applied by the actuator in order to hold the friction member firm. If in this state the actuator fails, the friction member is carried along unimpeded and the brake can lock suddenly. In the case of a motor vehicle wheel brake, this must be prevented at all events.
In order to prevent locking of the brake, various emergency release devices are known from the prior art. The known emergency release devices are, however, usually very complicated in structure and require a fairly large number of components.
Self-energizing brakes comprise in many cases also a lining-wear-readjustment device, by means of which the air gap between brake lining and brake disk, particularly in worn brakes, can be readjusted. Known readjustment devices usually contain their own actuator, e.g. an electric motor, and further components. As a result, the entire wheel brake with all auxiliary devices becomes very complex, elaborate and expensive and requires a relatively large amount of installation space.